The UK's carbon budgets are increasingly a matter of political strife. The consensus that prevailed among the three main parties on climate change before the last election has been repeatedly broken down by the right wing of the Conservatives, who see taking a sceptical stand on green issues as a key of appealing to the party's core voters.

Osborne has made no secret of his wish to loosen the UK's carbon budget.

But the preliminary conclusions of the committee, published on Thursday, only take account of the UK's carbon budget with respect to the government's ambitions for Europe-wide emissions targets, and the current circumstances with regard to greenhouse gas output across the EU.

Ministers will have to wait until December for the committee's full report, which will examine in detail the full domestic and international context for emissions reduction, and make a recommendation on whether the budget targets should stay at current levels.

Ministers could choose to ignore the advice of the committee, set up under the Climate Change Act, but to do so would provoke a major political row.

Many Tories would prefer to abolish the committee and repeal the act, but that would require a major revolt. Kennedy was in line to be appointed chief civil servant in charge of Decc, but this was vetoed personally by Cameron, for fear of howls of outrage from his backbenchers.

Kennedy wrote to the Lib Dem energy secretary, Ed Davey, on Thursday to relay the committee's latest findings. He wrote: "The assumptions regarding EU circumstances upon which the fourth carbon budget decision was made have not changed, and therefore there is no legal or economic justification to change the budget in this respect at this time. Rather, the budget remains cost-effective, with manageable costs and impacts, given our assessment of EU developments."

The government's official policy is to push in Brussels for the EU to adopt a tough new target of cutting emissions by 40% by 2030, compared with 1990 levels, and a target of 50% cuts if other countries outside the EU also up their efforts on climate change. Kennedy said: "Government should continue to push in the EU for an ambitious 2030 emissions target."

When the committee asked stakeholders for their views, they said that clarity on the UK's position within the EU's negotiations should be a top priority.

That is why the committee has published this part of its advice now, instead of waiting for its full assessment before the end of the year.

Kennedy wrote: "Responses to our recent call for evidence have suggested the need to clarify at an early stage the implications of European circumstances for the budget, which we are now able to do."

Friends of the Earth welcomed the report, with its executive director, Andy Atkins, saying: "This is a timely and welcome warning shot across the bows of the anti-green chancellor who's made no secret of his desire to water down the UK's commitment to tackling climate change."

The committee also found that the effects on competitiveness of sticking to the budget would be "limited and manageable."

Manufacturers' organisation, the EEF, said the committee was wrong on the issue of competitiveness. Its head of climate and environment policy, Gareth Stace, said: "The rest of the EU has not followed our lead and we are still waiting on agreement of the EU 2030 climate package, which is uncertain and delayed. We would therefore question how the committee can make an informed judgement now that the 2025 target will be in line with our EU and global competitors, when significant uncertainness remain."